The Enduring Mysteries of the Outer Solar System

Thefarthest reaches of our solar system remain the most mysterious areas aroundthe sun. Solving the mysteries of the outer solar system could shed light onhow the whole thing emerged — as well as how life on Earth was born.

Why the rainbow of colors in the Kuiper belt?

Forinstance, the Kuiper belt past Neptune is currently the suspected homeof comets that only take a few decades or at most centuries to completetheir solar orbits — so-called "short-period comets." Surprisingly, Kuiperbelt objects "show a wide range of colors — neutral or even slightly blueall the way to very red," said University of Hawaii astrophysicist David Jewitt.

Thecolor of an object helps reveal details about its surface composition. Itremains a mystery why Kuiper belt objects show a much wider range of color — andthus surface composition — than other planetoids, such as the asteroids.

Someresearchers had suggested volcanic activity could have led to all thesecolors — "absurd in the context of 100-kilometer-sized (60-mile) bodies,"Jewitt said, as volcanism needs something bigger.

Jewittand his colleagues had suggested that cosmic rays could have made Kuiper beltobjects redder, while impacts with rocks could have dug up more pristine matterthat made them less red. Nowadays Jewitt thinks there must be anotherexplanation for this rainbow, but it remains unknown.

What is ultra-red matter?

Thereappears to be a material dubbed "ultra-red matter" that exists onlyon about half of all Kuiper belt objects and their immediate progeny, known ascentaurs — icy planetoids orbiting between Jupiter and Neptune that very recentlyescaped from the Kuiper belt.

Thisultra-red matter does not exist in the inner solar system, "not even onthe comets which come from the Kuiper belt. This suggests that the ultra-redmatter is somehow unstable at the higher temperatures close to the sun," Jewittexplained.

"Inthe Kuiper belt objects, organics might have been 'cooked' by cosmic rayradiation, giving them dark red surfaces, but there is no proof," Jewittsaid. Ideally spacecraft could go out there and find out, he added.

Has the Kuiper belt shrunk?

Theoreticalcalculations suggest the Kuiper belt was once hundreds or maybe even thousandsof times more populated than it is now. "How was 99 percent or 99.9percent of the mass lost, and when?" Jewitt asked.

Oneconjecture suggests when Saturn and Jupiter shifted their orbits roughly 4billion years ago, their gravitational pulls slung Kuiper belt objects out ofthe solar system. Another says the Kuiper belt objects pulverized themselves todust, which then was swept away by the sun's radiation. Yet another possibility"is that we are missing something crucial and the conclusion that the beltis heavily depleted is wrong," Jewitt said. "All these possibilitiesare comparably hard to swallow, but would each be amazing, if true."

Secrets in the Oort cloud?

A distantreservoir of trillions of comets known as the Oort cloud theoretically lies upto 100,000 astronomical units from the sun — anastronomical unit or AU being about 93 million miles (150 million kilometers).This means the Oort cloud is a fifth of the way to the nearest star, sofar away that objects within it have never been seen directly, onlyinferred — but it must exist, given all the comets seen over the years.

The Oortcloud is the conjectured source of comets that require centuries or millenniato complete their long journeys around the sun. Since these "long-periodcomets" come from all directions, the Oort cloud is often thought to bespherical. However, while comets such as Halley's do not come from the Kuiperbelt, their orbits also do not jibe with a spherical Oort cloud, Jewittexplained. This suggests there may be an "inner Oort cloud" shapedkind of like a doughnut.

Astrophysiciststhink the Oort cloud is a remnant of the protoplanetary disk that formed aroundthe sun roughly 4.6 billion years ago. Learning more about the Oort cloud couldshed light on how our solar system — and Earth — were born, Jewitt said.

Arethere more dwarf planets?

So far,three dwarf planets are recognized — Ceres, Pluto and Eris. The Kuiper belt,which lies about 50 AU from the sun, could hold some 200 more. Beyond thatthere could be scores of dwarf-planet-sized bodiesbeyond roughly 100 AU from the sun "that nobody had seen before due totheir faintness and slow motion," said astronomer Chad Trujillo at GeminiObservatory in Hawaii. "Even a body as big as Mars could be missed in ourcurrent surveys if it were moved beyond a couple hundred AU."?

Thereare theories that the dwarf planets of the outer solar system may have dwelt inthe inner solar system billions of years ago, based on their current orbitaltrajectories. If so, "why are there so many ices on their surfaces?" Trujillo asked. Bodies in theinner solar system are generally expected to lose their ice due to sunlight.

Trujillo and his colleaguessuspect the ice now seen on these dwarf planets is relatively new, with suchreplacement ice coming perhaps from within these worlds, erupting out during"cryovolcanism." Of course, further research is needed to see if suchice renewal would be enough to cover the dwarf planet after they voyaged fromthe inner to the outer solar system, he added.

Do cosmic rays come from a bubble around the solar system?

Whenthe supersonic wind of charged particles that flows from our sun collides withthe thin gas foundbetween the stars, the solar wind essentially blows a bubble in this interstellarmedium — a ball known as the heliosphere.

Scientistshave thought unusually weak cosmic rays — energetic particles that zip fromspace at Earth — come from the heliosphere. Specifically, these rays are thought to comefrom the "terminationshock" — a shock wave of compressed, hot particles that results when thesolar wind abruptly brakes against interstellar gas. (The termination shockappears to be about 75 to 85 AU from the sun.)

However,Voyager 1 saw no sign these anomalous cosmic rays were produced at thetermination shock. "Perhaps it crossed the shock at the wrong time orplace," saidMIT astrophysicist John Richardson, or perhaps the standard view on how theseanomalous cosmic rays are generated is wrong. Voyager 2 crossed the terminationshock in 2007 about 10 billion miles away from where Voyager 1 crossed it in2004, and its data, which is still being analyzed, "may help us understandwhere these particle are produced," he explained.

"Cosmicrays have been reported to affect Earth's weather so understanding their sourceis important," Richardson added. Moreover, high-energy particles from shock waves triggeredby huge eruptions from the sun known as coronal mass ejections can damagespacecraft and astronauts, and better understanding the termination shock couldhelp understand these other, potentially dangerous particles.